Supporting base



FIG. 1 is a front elevational view of a supporting base showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a right side elevational view thereof, a left side elevational view thereof being a mirror image;

FIG. 4 is a top plan view thereof;

FIG. 5 is a bottom plan view thereof;

FIG. 6 is a front, top, right perspective view thereof; and,

FIG. 7 is a rear, bottom, right perspective view thereof.

The portions in broken lines are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a supporting base, as shown and described. 